A deep ALMA image of the Hubble Ultra Deep Field
Additional Document Info
2016 The Authors. We present the results of the first, deep Atacama Large Millimeter Array (ALMA) imaging covering the full 4.5 arcmin2 of the Hubble Ultra Deep Field (HUDF) imaged with Wide Field Camera 3/IR on HST. Using a 45-pointing mosaic, we have obtained a homogeneous 1.3-mm image reaching 1.3 35 Jy, at a resolution of 0.7 arcsec. From an initial list of 50 > 3.5 peaks, a rigorous analysis confirms 16 sources with S1.3 > 120 Jy. All of these have secure galaxy counterparts with robust redshifts ( z = 2.15). Due to the unparalleled supporting data, the physical properties of the ALMA sources are well constrained, including their stellar masses (M*) and UV+FIR star formation rates (SFR). Our results show that stellar mass is the best predictor of SFR in the high-redshift Universe; indeed at z 2 our ALMA sample contains seven of the nine galaxies in the HUDF with M* 2 1010M, and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow us to probe the redshift/mass distribution of the 1.3-mm background down to S1.3 10 Jy. We find strong evidence for a steep star-forming 'main sequence' at z 2, with SFR M* and a mean specific SFR 2.2 Gyr-1. Moreover, we find that 85 per cent of total star formation at z 2 is enshrouded in dust, with 65 per cent of all star formation at this epoch occurring in high-mass galaxies (M* > 2 1010M), for which the average obscured:unobscured SF ratio is 200. Finally, we revisit the cosmic evolution of SFR density; we find this peaks at z 2.5, and that the star-forming Universe transits from primarily unobscured to primarily obscured at z 4.